1. Field of the Invention
The present invention relates to chemical vapor deposition of films of ruthenium. More particularly, the invention relates to direct liquid injection of precursor solutions of ruthenium compounds for use in chemical vapor deposition of ruthenium and ruthenium oxide. The invention further relates to solvated ruthenium precursor formulations suitable for forming vaporized ruthenium precursors for use in chemical vapor deposition of ruthenium films onto substrates.
2. State of the Art
Use of chemical vapor deposition (“CVD”) methods for depositing a thin film of material on a substrate, such as a silicon wafer or other exposed material surface on a wafer or other semiconductor base, are widely known and used in the semiconductor fabrication industry. In typical CVD processes, a precursor, such as a heat decomposable volatile compound, is contacted with a substrate which has been heated to a temperature above the decomposition temperature of the precursor. In this fashion, a coating, which typically comprises a metal, metal mixture or alloy, ceramic, metal compound, or a mixture thereof, depending on the choice of precursors and reaction conditions, is formed on the substrate.
Use of CVD as a thin film formation method includes numerous desirable characteristics, such as the ability to readily control the composition of the thin film and the ability to form a thin film without contamination of, or damage to, the substrate. CVD may also be used to deposit films of metals into vias, trenches, and other recesses or stepped structures. In situations where conformal thin-film deposition is required, CVD techniques are a preferred method of deposition, since evaporation and sputtering techniques cannot be used to form a conformal thin-film deposition layer.
While CVD techniques have been described in the literature with reference to many metals and metalloids, commercial use of CVD has been predominantly confined to deposition of a few metals and metal compounds, such as tungsten, silicon, titanium nitride, silicon oxide, iron, and aluminum. CVD of other metals has been limited due to a variety of reasons, including formation of poor film quality, requirement of high processing temperatures, lack of suitable precursor compounds, and instability of the precursors used in the deposition systems. The availability of suitable volatile and heat-decomposable precursor compounds appears to be a significant limiting factor in the application of CVD to the production of metal-containing films.
In integrated circuit processing, selected precursor compounds have been used to form conducting films that can maintain their integrity at elevated temperatures. Ruthenium and ruthenium dioxide (RuO2) are particularly well-suited as conducting films for such applications since they have good electrical conductivities, exhibit high stability over a wide temperature range and exhibit good adherence to silicon, silicon dioxide, and ceramic oxides. Films of ruthenium and ruthenium oxide deposited by CVD have been proposed to be useful for contact metallizations, diffusion barriers, and gate metallizations. M. L. Green et al., J. Electrochem. Soc., 132, 2677 (1985).
There are a wide variety of ruthenium compounds that can be used as precursors for the preparation of such films. Many are particularly well suited for use in chemical vapor deposition techniques. For example, U.S. Pat. No. 5,372,849 to McCormick et al. discloses the use of ruthenium compounds containing carbonyl ligands and other ligands. However, such compounds are typically less volatile and not as easily used in chemical vapor deposition techniques.
Another use of ruthenium precursors for the preparation of films involves use of a chemical spray deposition process wherein tris(acetylacetonate)ruthenium in butanol is converted into an aerosol spray using a hydrogen/nitrogen mixture as the carrier gas. Triruthenium dodecacarbonyl, ruthenocene, and tris(acetylacetonate)ruthenium have also been compared as CVD precursors in the formation of ruthenium and RuO2 by M. Green et al., in J. Electrochen. Soc., 132, 2677 (1985). However, because none of the aforementioned precursors are very volatile, high deposition rates using these precursors are difficult to obtain.
U.S. Pat. No. 4,250,210, issued Feb. 10, 1981 to Crosby et al., discloses the use of ruthenium 1, 3 dione compounds, such as tris(acetylacetonate)ruthenium and its fluorinated derivatives, in the CVD of ruthenium films. Although the fluorinated ligands are said to provide greater volatility and good deposition rates when heated to over 200° C., difficulties in attaining uniform coatings are noted due to the poor stability of the precursors. Furthermore, organic byproducts with very low vapor pressures are formed and collect in the reactor during the volatilization process, which can create a serious contamination problem in production-scale applications of the tris(acetylacetonate)ruthenium precursors.
Also disclosed in the Crosby patent is the use of ruthenium carbonyl chloride and penta(trifluorophosphine)ruthenium as precursors for ruthenium CVD. Use of these precursor compounds, however, is undesirable because the obtainable rates of deposition of ruthenium are very low and ruthenium carbonyl chloride corrodes certain substrates, making a consistent product preparation difficult or impossible.
In view of the described shortcomings, it would be useful to utilize ruthenium precursor formulas having both high stability and high volatility that are easy to prepare and use in low temperature CVD processes and which are capable of depositing high-quality, continuous films of ruthenium having good surface morphology. While many ruthenium precursor compounds possessing such characteristics are known, those same compounds typically have freezing points around room temperature. Thus, in order to prevent these ruthenium precursor compounds from freezing, the system typically needs to be heated. Using a solution of ruthenium precursor compounds in an organic solvent instead of the neat precursor for a liquid delivery system would eliminate the necessity of heating the direct liquid injection (DLI) system. These same precursor compounds, however, are usually very temperature sensitive, with heating at slightly elevated temperatures resulting in the decomposition of the precursor. Side products of this decomposition are solids, which are detrimental to a liquid delivery process as well. Also measuring and controlling extremely small amounts (e.g., microliter/min) of ruthenium precursors is very difficult.
Thus, in view of the shortcomings of the available precursors, a continuing need exists for improved ruthenium precursor formulations useful for the CVD of films of ruthenium and ruthenium-containing films (e.g., RuO2, SrRuO3, RuSix). More specifically, a need exists for high-volatility ruthenium precursor formulations that are highly stable, maintain a ruthenium precursor in a soluble state, and do not require heating of the precursor formulation prior to introducing the same into a CVD system. The precursor formulations should also be easy to prepare, easy to measure, convenient to use in low-temperature CVD processes, and available in more manageable delivery forms (i.e., ml/min rate).